Tips & Tricks
As described in the YouTube defect calculation tutorial, our
recommended workflow for calculating interstitial defects is to first generate the set of
possible interstitial sites for your structure using
DefectsGenerator (which uses Voronoi tessellation
to do this), and then perform Gamma-point-only relaxations (using
vasp_gam) for the neutral state of
each generated interstitial candidate. The
vasp_gam relaxation files can be generated following the
steps shown in the
defect generation tutorial and
vasp_gam = True in
We can then compare the energies of these trial neutral relaxations, and remove any candidates that either:
Are very high energy (>1 eV above the lowest energy site), and so are unlikely to form.
Relax to the same final structure/energy as other interstitial sites (despite different initial positions), and so are unnecessary to calculate. This can happen due to interstitial migration within the relaxation calculation, from an unfavourable higher energy site, to a lower energy one. Typically if the energy from the test neutral vasp_gam relaxations are within a couple meV of eachother, this is the case.
Difficult Structural Relaxations
If defect supercell relaxations do not converge after multiple continuation calculations
POSCAR and resubmitting the job), this is likely due to small
residual forces causing the local optimisation algorithm to struggle to find a solution, an error in the
underlying calculation and/or extreme forces.
If the calculation outputs show that the relaxation is proceeding fine, without any errors, just not converging to completion, then it suggests that the structure relaxation is bouncing around a narrow region of the potential energy surface. Here, the gradient-based geometry optimiser is struggling to converge.
Often (but not always) this indicates that the structure may be stuck around a saddle point or shallow local minimum on the potential energy surface (PES), so it’s important to make sure that you have performed structure-searching (PES scanning) with ShakeNBreak (
SnB) to avoid this. You may want to try ‘rattling’ the structure to break symmetry in case this is an issue, as detailed in this part of the
Alternatively (if you have already performed `SnB` structure-seaerching), convergence of the forces can be aided by:
Switching the ionic relaxation algorithm back and forth (i.e. change
Reducing the ionic step width (e.g. change
Switching the electronic minimisation algorithm (e.g. change
All), if electronic convergence seems to be causing issues.
Tightening/reducing the electronic convergence criterion (e.g. change
If instead the calculation is crashing due to an error and/or extreme forces, a common culprit is the
EDWAVerror in the output file, which can often be avoided by reducing
KPAR. If this doesn’t fix it, switching the electronic minimisation algorithm (e.g. change
All) can sometimes help.
If some relaxations are still not converging after multiple continuations, you should check the calculation output files to see if this requires fixing. Often this may require changing a specific
INCARsetting, and using the updated setting(s) for any other relaxations that are also struggling to converge.
For tips on the
ShakeNBreak part of the defect calculation workflow, please refer to the
Have any tips for users from using
doped? Please share it with the developers and we’ll add them here!